When your Funding Wallet balance is insufficient to pay for a transaction, the system will automatically use the funds from your Spot Wallet to pay for the full amount of the transaction. However, if your Spot Wallet balance is also insufficient, the transaction will be declined.
if your Spot Wallet balance is also insufficient, the system will automatically use the corresponding funds from your Flexible Savings
I personally would use more the binance card knowing that I could reap the benefits of the Flexible Savings, e.g. BUSD